Permeability and self-healing of cracked concrete as a function of temperature and crack width

Tests have been carried out on a high-strength concrete establishing permeability and self-healing behaviour of cracked concrete as a function of temperature between 20 and 80 °C and crack width between 0.05 and 0.20 mm. The results show a considerable increase of water transport with temperature. Theoretical prediction on the basis of thermodynamics showed reasonable to good agreement between theory and experiment.     

Influence of shrinkage and water transport mechanisms on microstructure and crack formation of tile adhesive mortars

Shrinkage and expansion of cementitious materials like tile adhesive mortars depend on the presence of water as well as on the drying and rewetting history. Particularly for large-sized tiles such volumetric changes induce stress concentrations, which may result in cracking. This study focuses on the interplay between water infiltration and cracking, starting at the early curing of the mortar during the first days after application until water transport in the hardened system. Based on laboratory experiments, different events concerning the effect of water transport were induced by variation of the experimental setups in order to provoke cracking. Cracks at the tile–mortar interfaces suggest these domains to reflect zones of mechanical weakness. Along these cracks, water can enter the system inducing precipitation of secondary minerals in cracks and pores already after one wetting cycle. These processes reveal increasing importance during repeated cycles of drying and wetting, i.e. under outdoor conditions and may lead to failures.     

Strength, pore structure and permeability of alkali-activated slag mortars

This paper deals with the strength development, pore structure development, rapid chloride permeability and water permeability of alkali-activated slag mortars activated by 6% (by mass of Na₂O) NaOH, Na₂CO₃ and Na₂SiO₃. The Na₂SiO₃-activated slag mortars exhibited the highest strength at both early and later ages, even much higher than a typical commercial Type III portland cement. NaOH-activated slag mortars exhibited the lowest strength. The pore structure measurements were consistent with strength results. Four common strength-porosity equations: Balshin's, Ryshkevitch's, Schiller's, and Hasselmann's equations, fit the experimental results from alkali-activated slag mortars with sufficient efficiency; of which Hasselmann's equation fit best. The charge passed through the mortar specimens in the rapid chloride ion permeability test appeared to be dependent more on the chemistry of pore solution than on the pore structure of the mortars. Limited results from water permeability testing appeared to be consistent with strength and pore structure measurements.     

Crack effects on gas and water permeability of concretes

The relationship between load-induced cracking and concrete permeability is studied. Ordinary concrete (OC) and high-performance concrete (HPC), including steel fiber-reinforced concrete (HPFRC), are used. Two discs, 50 mm-thick slices, cut from 110-220 mm cylindrical specimens are diametrically loaded, as for a normal splitting test. The lateral displacement, also called the crack opening displacement (COD) is monitored for each loading cycle. After unloading, gas and finally water permeability tests are both performed, using constant head permeameter, to compare the influence of the percolating fluid and the COD. Due to the wide range of measured gas flow, Klinkenberg's and Dupuit-Forcheimer's laws are applied to compute the intrinsic gas permeability. Results suggest it increases proportionally to the cube of the COD and it matches water permeability, if only the first water percolating time is considered. The roughness parameter of the cracks induced in each concrete, is compared and discussed.     

Effects of organo-modified montmorillonite on strengths and permeability of cement mortars

Organo-modified montmorillonites (OMMT) which have been widely used in polymer/clay nano-composites are employed here as fillers and reinforcements in cement mortars. The ratio of quartz sand and cement is 2.75 while the water/cement ratios of 0.40, 0.485 and 0.55 are considered for the cement mortars we studied. Experimental results indicate that the coefficients of permeability of cement mortars could be 100 times lower if a lower dosage of OMMT micro-particles is added. At the same time, the compressive and flexural strengths of cement mortars can be even increased up to 40% and 10%, respectively. It is also found that the optimal dosage of OMMT micro-particles to give higher compressive and flexural strengths and a lower coefficient of permeability for cement mortars is less than 1%. Meanwhile, the microstructure of cement mortars is characterized by using SEM, EDS and MIP to evaluate the effects of OMMT micro-particles on the improvements of strengths and permeability of cement mortars.     

Real-time water permeability evolution of a localized crack in concrete under loading

This paper presents a real-time relation between the evolution of fluid permeability and the crack width opening in a saturated concrete sample. The standard Brazilian test has been enhanced so that one single controlled crack (up to 300 μm) is generated in the specimen. The procedure has been then adapted so that the simultaneous monitoring of the cracking process and the evolution of a fluid flow through the specimen are possible. Poiseulle's cubic law is commonly used in numerical analyses for describing the percolation of a fluid in a crack: this law is then corrected accordingly to our experimental observations thus taking into account the actual morphology of the opening crack. The effect of the size of the specimen (diameter and thickness) on the percolation process as well as threshold effect are also investigated and discussed.     

Influence of mix proportions on microstructure and gas permeability of cement pastes and mortars

This paper presents a study on the influence of mix proportions of cementitious materials on their transfer properties, namely porosity and gas permeability. These latter are known as durability indicators. The work is performed on a wide range of cement pastes and mortars (24 compositions). These compositions are defined by mix proportion parameters (water/cement ratio, limestone filler/cement ratio, and amount of superplasticizer and volume fraction of paste). To characterize these materials, an experimental campaign was carried out, including different types of test (water porosimetry, mercury intrusion porosimetry, desorption isotherms and gas permeability). The influence of the composition parameters on the studied durability indicators is highlighted and correlation between gas permeability and microstructural properties (total porosity and critical pore diameter) is established. Finally, a method to predict materials permeability from that of the cement paste is proposed.     

Effect of crack opening on the local diffusion of chloride in cracked mortar samples

The paper presents an experimental study on chloride penetration in cracked mortar specimens. A mechanical expansive core was used to generate cracks of constant width across the thickness of the sample. Sixteen specimens with crack openings ranging from 6 to 325 µm were subjected to a test designed to allow chloride diffusion along the crack path for a period of 14 days. Chloride penetration tests were carried out on mortars at 28 days and 2 years. Relationships between crack opening and chloride–ion diffusion along a crack are presented and discussed. The results show that crack opening significantly affects chloride–ion diffusion along a crack. Overall, chloride diffusion along a crack decreases with crack opening. On the other hand, no chloride diffusion occurs in cracks with an opening of 30 µm or less. This crack-opening threshold agrees with the critical crack opening obtained from a stress-displacement curve of a mortar sample subjected to uniaxial tension. At crack openings greater than the threshold value, chloride diffusion along the crack path depends on mortar age. This result suggests that self-healing could reduce chloride diffusion in cracks.     

Estimating the permeability of cement pastes and mortars using image analysis and effective medium theory

A method to estimate permeability of cement-based materials using pore areas and perimeters from SEM images is presented. The pore structure is idealised as a cubic lattice having pores of arbitrary size. The hydraulic conductance of each pore is calculated using the hydraulic radius approximation, and a stereological factor is applied to account for the random orientation of the image plane. A ‘constriction factor’ is applied to account for variations in pore radius along the pore axis. Kirkpatrick's effective medium equation is then used to obtain an effective pore conductance, from which the macroscopic permeability is derived. The method was tested on forty-six pastes and mortars with different w/c ratio, cement, age and sand content. The permeabilities ranged from 3 × 10^? 18 m² to 5.8 × 10^? 16 m². It was found that 76% of the permeabilities were predicted to within a factor of ± 2, and 98% within a factor of ± 5 from measured values.     

Effects of metakaolin, water/binder ratio and interfacial transition zones on the microhardness of cement mortars

Variations in the microhardness of the hydrated cement matrix component of model mortars have been investigated as functions of the distance from the aggregate surfaces for specimens in which the binder was Portland cement or a blend of Portland cement and metakaolin.Microhardness measurements were made using a Knoop indenter at distances of up to 120 μm from the aggregate. The microhardnesses of the paste-aggregate interfacial transition zones (ITZs) were found to be between 14% and 22% lower than those of the corresponding bulk cement pastes at the lower water/binder ratios investigated, i.e. 0.4 and 0.5 for samples prepared with Portland cement and 0.4 for samples prepared with a binder comprising Portland cement and metakaolin.Metakaolin increased the mean microhardness of specimens prepared at the higher water/binder ratios of 0.5 and 0.6 by 13% and 54%, respectively.     

Cellulose ethers influence on water retention and consistency in cement-based mortars

Cellulose ethers (CE) are commonly used as additives to improve the quality of cement-based materials. As admixtures, they improve the properties of mortars such as water retention, workability, and open time. Also, polysaccharides such as starch derivatives are used to improve the consistency of the fresh material.The properties of cement-based mortars at fresh state were investigated. The effect of CE and their physico-chemical parameters (molecular weight, substitution degrees, etc.) on both water retention and rheological properties of mortars were studied. Moreover, some starch derivatives were also examined in order to better understand the water retention mechanisms.Rheological measurements showed that CE have a thickening effect for a content of 0.27 wt.%. Besides, a fundamental effect of CE molecular weight on mortar consistency and its water retention capability was highlighted. Finally, the comparison with starch ethers proved that, for those admixtures, water retention is not directly linked to mortar's viscosity.     

Modeling the effects of solution temperature and concentration during sulfate attack on cement mortars

Simple chemistry-based empirical models have been developed to assess the role of temperature and concentration of the sulfate solution in the process of expansion of cement mortars that are subjected to external sulfate attack. ASTM Type I PC mortars, prepared according to ASTM C-109, were immersed in sodium and magnesium sulfate solutions at five different concentrations and four different temperatures. For both solutions, the trends in the measured expansion suggested the use of a simple rate law to analyze the effect of concentration. For the effect of temperature, an Arrhenius relationship was developed to determine the activation energy required to initiate expansion in sodium sulfate solution. Regression-based statistical models were found to be sufficient to explain the effect of temperature of magnesium sulfate solution on the expansion. Implications of using these models for developing potential test methods, as well as to enable interpretation of data from nonstandard test methods, are discussed.     

Influence of the kneading water content in the behaviour of single-coat mortars

The effect of water content in the kneading process of mortars for application as single-coat renders was evaluated in terms of several properties. These properties refer to the mortar product in the fresh and hardened state. Characteristics under evaluation in function of kneading water are, for fresh mortar, the apparent density and entrained air while, for hardened products, apparent density, shrinkage, mechanical properties, abrasion wear and permeability amongst others. If some of these characteristics are obtained by standard tests, others, like abrasion wear, had to be adapted and are described here.Both for fresh and hardened products characteristics, kneading water amount significantly changes them and it was possible to establish the proper content of water to be used in order to get a well-behaved single-coat mortar.     

Surface roughness effects on the stress analysis of adhesive joints

A finite element program was used to perform a parametric analysis of stresses generated by adherend surface roughness in lap and butt joints. Roughness asperities were idealized as having tip radius of curvature R1, height H1, and slopes A1. Failure criteria (maximum stress, yield, and fracture) were related to the geometric parameters by functions of the form: $\text{σ = σ}{}_{\mathrm{o}}\text{+ cR}{}^{\mathrm{r}}\text{H}{}^{\mathrm{h}}\text{A}{}^{\mathrm{a}}$The specific material properties used for the analysis were those of tooth enamel and an orthodontic cement. Qualitatively however, these results are applicable to all structural adhesives.     

The effects of water and frequency on fatigue crack growth rate in modified and unmodified polyvinyl chloride

A study of the influence of water environments on the cyclic fatigue crack behavior of polyvinyl chloride (PVC), with (PVC‐M) and without (PVC‐U) chlorinated polyethylene (CPE) impact modifier was undertaken and compared with corresponding results in air. Two frequencies of 1 and 7 Hz were applied to assess the influence of frequency on the fatigue behavior; a higher fatigue resistance and threshold were obtained with increasing frequency. This trend is more significant in water. However, in this environment, the fatigue resistance deteriorated under conditions of higher stress intensity factor amplitude (ΔK) and frequency. The fatigue properties of PVC‐U are the most affected by the presence of water, particularly at low frequency and higher ΔK. Examination of the fracture surface showed the interaction of water molecules and the PVC matrix with the formation of (1) a nodular structure, close to the fatigue threshold and (2) plasticized structures at high ΔK, which are associated with a greater threshold value and fatigue resistance. The absorption of the water retarded the fibrillation of craze and caused crack blunting effects. Water functions as a plasticizer, particularly at high ΔK, through the formation of the plasticized structures. Results are compared with those observed from an in‐service failure. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

裂解气压缩机级间注水技术的应用

介绍了裂解气压缩机由注洗油改为注水情况，进而得出结论：改注水降温效果明显，压缩机工作效能可得到有效保障。     

Draft angle and surface roughness effects on stereolithography molds

Stereolithography (SL) is a rapid prototyping process, which allows one to build complex shapes quickly. Current research investigates the possibilities of using this process to make injection molds. This would allow designers to manufacture and test molds easily and rapidly. One of the main issues with this technique is the effects of its surface on the part. Molds built by SL have high roughness. This gives rise to a high friction force between the part and the mold, and increases the ejection force needed to eject the part from the mold. High ejection forces often lead to damage or breakage of the part and the mold. Research was undertaken on the effects of draft angle and roughness on ejection forces. It was found that increasing the draft angle does not necessary assist the ejection of the part. As the draft angle increases, the roughness and hence the friction force between the part and the mold also increase. There is a trade‐off between draft angle and roughness. A model based on Glanvill's equation was developed to predict ejection force and was consistent with experimental results.     

The effects of soft segments on the physical properties and water vapor permeability of H12MDI-PU cast films

This research was based on the study of the effects of H₁₂MDI-1, 4BD PU soft segments on the physical properties and water vapor permeability of films cast from solvent evaporation or wet coagulation method. The soft segments studied included polyether, polyester, and polycaprolactone polydiols. The NCO/OH mol ratios of prepolymer were prepared by 2, 3, 4, 5, and 8, respectively. The chain lengths of the soft segments used were: PTMG of molecular weights 650, 1000, 2000, and 2900; PBA of 1000, 2000, and 3000. The results revealed that the polyether-based PU cast films had lower T_gs than the polyester-based PU films. In general, the polyether-based PU films shows the characters of higher water vapor permeability, lower breaking elongation, and higher breaking strength. Films with higher molecular weight soft segments in the polymer chains exhibited lower T_gs, lower breaking strength, higher breaking elongation, and higher water vapor permeability. As the hard segment contents were increased, the films exhibited higher T_gs. Films with higher hard-segment ratios had the highest breaking strength but the water vapor permeability, on the other hand, became lower. Films cast from the solvent evaporation method had higher breaking strength and higher breaking elongation but lower water vapor permeability than films cast from the wet coagulation method. © 1994 John Wiley & Sons, Inc.     

Effect of crack width on chloride diffusion coefficients of concrete by steady-state migration tests

The purpose of the present study is to explore the diffusion characteristics of cracked concrete according to the width of cracks. Major test variables include crack width, concrete strength, fly ash addition, and maximum aggregate size. The diffusion characteristics have been measured by steady-state migration test. The present study indicates that the diffusion coefficients do not increase with increasing crack widths up to the so-called “threshold crack width.” The threshold crack width for diffusion is found to be around 55–80 μm. Above this threshold value, the diffusion coefficients start to increase with crack width. A composite model with the introduction of “crack geometry factor” was derived to identify the diffusion coefficient in cracked concrete. It was shown that the crack geometry factor ranges from 0.067 to 0.206. Finally, the effects of concrete strength, fly ash addition and maximum aggregate size on diffusion coefficients are also discussed.     

Quantifying the effects of contact duration, loading rate, and approach velocity on P-selectin-PSGL-1 interactions using AFM

Kinetics and its regulation by extrinsic physical factors govern selectin-ligand interactions that mediate tethering and rolling of circulating cells on the vessel wall under hemodynamic forces. While the force regulation of off-rate for dissociation of selectin-ligand bonds has been extensively studied, much less is known about how transport impacts the on-rate for association of these bonds and their stability. We used atomic force microscopy (AFM) to quantify how the contact duration, loading rate, and approach velocity affected kinetic rates and strength of bonds of P-selectin interacting with P-selectin glycoprotein ligand 1 (PSGL-1). We found a saturable relationship between the contact time and the rupture force, a biphasic relationship between the adhesion probability and the retraction velocity, a piece-wise linear relationship between the rupture force and the logarithm of the loading rate, and a threshold relationship between the approach velocity and the rupture force. These results provide new insights into how physical factors regulate receptor-ligand interactions.